Electrochemical sensors are often used for determining concentrations of certain substances in a measured medium, both in laboratory measurements technology as well as also in process measurements technology, in many fields of chemistry, environmental analysis, biochemistry, biotechnology, pharmacy, food technology and water management. Electrochemical sensors of the field of the invention can be, for example, potentiometric or amperometric sensors. Potentiometric sensors comprise, as a rule, a measuring half-cell, which, in contact with the measured medium, forms a potential dependent on the concentration of the analyte in the measured medium, a reference half-cell, which, in contact with the measured medium, outputs a potential independent of the analyte concentration to be determined, as well as a measuring circuit, which produces a measurement signal representing the potential difference between the measuring half-cell and the reference half-cell and, in given cases, outputs such to a superordinated unit connected with the sensor, for example, a superordinated unit in the form of a measurement transmitter.
The measuring half-cell can, depending on the type of potentiometric sensor, comprise, as sensorially active component, for example, a redox electrode, an analyte sensitive coating or an ion-selective membrane. Forming a special case of ion selective membranes are pH-sensitive, glass membranes, which serve as the sensorially active component of potentiometric pH-sensors.
The reference half-cell of a potentiometric sensor is frequently embodied as an electrode of second type, for example, as a silver/silver chloride electrode (Ag/AgCl electrode). Such a reference half-cell has a reference electrolyte accommodated in a housing of an electrically insulating material, into which a chloridized silver wire extends. The reference electrolyte is, for example, a 3 molar KCl solution. Arranged in the housing wall is a liquid junction, for example, a passageway, a window of ground glass or a porous diaphragm, through which the reference electrolyte is in ionically conducting contact via a liquid-liquid interface with a surrounding medium, for example, the measured medium.
Amperometric sensors can comprise, for example, a three electrode circuit, including a working electrode, a counter electrode and a non-current carrying, reference electrode. The reference electrode, through which electrical current does not flow, referred to here and in the following likewise as a half-cell, can be embodied in equal manner as a reference half-cell of a potentiometric sensor, as an electrode of second type.
The liquid junction of a reference half-cell of the field of the invention is frequently implemented by a porous diaphragm of a ceramic material, glass or Teflon, which is introduced into the housing wall of the reference half-cell, for example, by being welded in or held in place by adhesive. The ceramic material is, above all, a zirconium dioxide ceramic. The porous structure, e.g. the number and size distribution of the pores, influences properties, such as the electrolyte outflow or the impedance of the liquid junction, decisively. Moreover, especially in the case of measuring in media with lesser conductivity, diffusion- and streaming potentials lead to measurement uncertainties.
In the article by Wolfgang Knappek entitled “Neue pH-Messketten für Labor und Prozess (New pH-measuring chains for laboratory and process)”, GIT Labor-Fachzeitschrift (Laboratory Journal), September/2001, Pgs. 2-4, pH combination electrodes are described, whose reference half-cell has a platinum diaphragm as liquid junction. A platinum diaphragm is composed of a plurality of platinum wires twisted together, between which are formed hollow spaces serving as ducts, which connect the electrolyte filled, housing interior of the reference half-cell with the measured medium. The reference electrolyte flows through the ducts with uniform velocity. In this way, the diaphragm is rinsed clean at the contact with the measured solution. As a result of these characteristics, a potentiometric pH sensor having such a reference half-cell is said to possess a very short settling time to steady measured values, a high accuracy of measurement and insensitivity to influences of stirring and flowing liquid.
Disadvantageous in the case of platinum diaphragms as liquid junctions for electrochemical sensors is, however, on the one hand, the relatively high manufacturing costs and the, in comparison to conventional ceramic-, glass or Teflon diaphragms, clearly higher loss of reference electrolyte through the diaphragm during operation. This is especially disadvantageous in process applications, in which a measuring chain should work stably over longer periods of time.